When a water droplet contacts a solid surface, a contact angle range of 70° to 150° where the water droplet contacts the surface is defined as hydrophobicity, and a contact angle range of 150° or more where the water droplet contacts the surface is defined as superhydrophobicity. In particular, on a solid surface having a water contact angle of 170° or more, water droplets roll off the surface and a clean state is maintained for a long time without leaving a trace of contact with water. In other words, it is expected that even after an aqueous solution is allowed to flow into a container or the like having a superhydrophobic surface, a completely clean state can be maintained without leaving even droplets of the solution on the inner wall of the container, and the container can be repeatedly used without washing. However, it is difficult to exhibit superhydrophobicity using only a molecule residue having a low surface tension.
On the other hand, many living matters of the natural world exhibit superhydrophobicity. For example, lotus leaves, rice plant leaves, cabbage leaves, and the like have superhydrophobicity in which water droplets are completely repelled. For example, it is known that the superhydrophobicity of a lotus leaf is deeply related to the surface structure of the leaf. Specifically, nanofibers extend throughout the entire surface to form a surface layer, micron-sized projections similar to aggregates of nanofibers make up an outermost layer on the surface layer at certain intervals, and hydrophobic wax is present on the surfaces of these nanofibers. As a result, water cannot adhere to the surface and rolls off the surface of the lotus leaf, and surface contaminations and the like are removed by the force of rolling, thus exhibiting a so-called self-cleaning function. This suggests that, in order to exhibit superhydrophobicity, it is most important to control the surface roughness, that is, the surface structural object and shape in the nano-dimension.
The structural principle for exhibiting superhydrophobicity, which is also called “lotus effect”, has been used as a guideline for developing many methods for designing an artificial lotus-like structure. With the advancement of nanomaterials, in recent years, various techniques for providing a flat solid surface that exhibits superhydrophobicity have been developed. For example, it has been reported that the contact angle is increased to 170° or more by regularly arranging carbon nanotubes on a surface of a substrate (refer to NPL 1). Is has also been reported that the surface contact angle is controlled to be 170° or more by growing polypyrrole nanofibers on a silicon surface coated with platinum by an electrochemical process (refer to NPL 2). Furthermore, superhydrophobicity is exhibited by forming a nanocrystal seeds layer composed of zinc oxide on a surface of a glass substrate at a temperature of 400° C. or higher, and then growing a large number of rod-like zinc oxide nanofibers thereon (refer to NPL 3).
As for a simple process, for example, it has been reported that a network structure composed of polypropylene nanoparticles is formed by adding a certain poor solvent to a solution of polypropylene, casting the resultant mixture onto a surface of a substrate, and then adjusting the temperature, whereby the contact angle is increased to 160° (refer to NPL 4). Alternatively, superhydrophobicity can be exhibited by providing glass made of oxides of silicon, boron, and sodium with a phase separation structure, and etching the glass by a chemical treatment to induce an irregular structure on the surface, and lastly allowing a fluorine compound to react on the surface (refer to PTL 1). Furthermore, it is also known that a superhydrophobic surface is constructed by preparing a stacked film of a polyarylamine and polyacrylic acid, and then treating a surface of the film by a chemical method to induce a surface porous structure, immobilizing silica nanoparticles thereon, and then lastly hydrophobing with a silane coupling agent having a fluorine alkyl residue (refer to PTL 2).
Among the processes proposed above, in the cases of a superhydrophobic surface based on an inorganic material, the step of obtaining a surface roughness provided with a nanostructure is complex, and the cost is also high. In the cases of a superhydrophobic surface based on an organic polymer, although the cost is low, the solvent resistance and corrosion resistance of the resultant superhydrophobic surface are low, and furthermore, the step of obtaining a surface roughness provided with a nanostructure is also complex. Thus, there is a problem in terms of practical use. Furthermore, the processes proposed above are characterized in that the superhydrophobic surface is formed on a planar solid surface. However, no example has been reported in which a limited space, specifically, an inner wall of tubular substrate is coated with an inorganic material-based superhydrophobic surface.
A prerequisite for exhibiting superhydrophobicity is to construct a nanostructure composite surface in which micro-sized domains are scattered over the entire surface of a substrate while fibrous nanostructures densely cover the surface of the substrate. However, many studies are still necessary to ascertain which materials and which processes are beneficial for constructing such a nanostructure composite. In particular, even when only the contact angle is temporarily high and superhydrophobicity is exhibited, the superhydrophobicity often disappears after a prolonged immersion in water. Considering that exhibition of superhydrophobicity for only a short period of time does not have practical utility, developing a material that exhibits semi-permanent or permanent superhydrophobicity (super-water repellency) even while being immersed in water, and providing a simple process for producing such a material are important objectives.